The present disclosure relates to a two-wire field device, and particularly to a technique for enhancing noise resistance of a two-wire field device having a sensor portion.
A two-wire field device having a sensor portion for measuring a physical amount is configured to be operated by a constant voltage supplied from a direct current power supply device through a loop wiring and also to convert a measuring result of the sensor portion to a direct current value to output the value to the loop wiring. Recently, a technique in which a direct current signal overlapped with digital data is transmitted and received is widely performed.
In Patent Document 1, as shown in FIGS. 6A and 6B, in a field device having an effective capacitance Ceff, in order to prevent charges accumulated in the capacitance from being discharged into a loop wiring, a diode D as a rectifying element is connected in series to an input side (FIG. 6A) or to a plus side and a minus side (FIG. 6B). Also, in order to escape a high frequency noise, which flows through the loop wiring, to the ground, capacitors Ch and Cl are respectively connected to the plus side and the minus side. Meanwhile, the diode D is configured in three stages for a redundancy structure, but may be configured in one stage.
[Patent Document 1] International Patent Publication No. WO95/34027
FIGS. 7A and 7B are views schematically showing an impedance circuit including the effective capacitance Ceff by means of circuit blocks of an actual field device. Namely, the field device has a current output circuit 310, a power supply circuit/control circuit 320 and a sensor circuit 330. Meanwhile, elements of a direct power supply system connected to the loop wiring are omitted.
FIG. 7A is a case where a diode 340 as a rectifying element is connected to a plus side of a terminal portion, corresponding to FIG. 6A. FIG. 7B is a case where a diode 340 is connected to a plus side of a terminal portion and a diode 350 is connected to a minus side of the terminal portion, corresponding to FIG. 6B.
The current output circuit 310 controls a current to be flowed into the loop wiring. The power supply circuit/control circuit 320 includes a power supply circuit for supplying electric power, which has been supplied thereto through the loop wiring, to each circuit and a control circuit for calculating a signal from the sensor circuit 330 and determining a current to be flowed into the loop wiring. Impedance Za of the current output circuit 310 is designated as Za and impedance of the power supply circuit/control circuit 320 is designated as Zb.
The sensor circuit 330 measures a physical amount and transmits the physical amount to the power supply circuit/control circuit 320. Herein, the sensor circuit 330 is a grounding type in which a reference point thereof is grounded. Accordingly, impedance Zsh (plus side) and impedance Zsl (minus side) exist with respect to the ground due to stray capacitance and the like.
In the field device, a parallel circuit, which is constituted of the power supply circuit/control circuit 320 and the sensor circuit 330, and the current output circuit 310 are generally connected to the loop wiring in a state that the parallel circuit and the current output circuit 310 are connected in series with each other, and the current output circuit 310 is arranged on the plus side.
Herein, it is assumed that a common-mode noise Vn, which is a noise having an identical phase on the plus side and the minus side and measured with respect to the ground, is mixed in the loop wiring of the field device. The common-mode noise Vn is occurred due to electromagnetic interference, such as unnecessary radiation.
The common-mod noise Vn influences the plus side of the sensor circuit 330 by means of a path A extending from the loop wiring through the current output circuit 310 to the plus side of the sensor circuit 330 and also influences the minus side of the sensor circuit 330 by means of a path B extending from the loop wiring to the minus side of the sensor circuit 330.
In the path A of the circuit of FIG. 7A, the common-mode noise Vn is rectified by the diode 340 and the capacitor Ch, and also a noise divided by the impedance Za of the current output circuit 310 and the plus-side impedance Zsh of the sensor circuit 330 is applied to the plus side of the sensor circuit 330. Because the impedance Za of the current output circuit 310 is generally large, an influence of the noise on the sensor circuit 330 is relatively small.
On the other hand, in the path B of the circuit of FIG. 7A, a component of the common-mode noise, which cannot be removed by the capacitor Cl, is directly applied to the sensor circuit 330. Accordingly, noise resistance of the sensor circuit 30 is deteriorated.
Contrarily, in the circuit of FIG. 7B, a path A is the same as that of FIG. 7A, but a path B is configured so that a noise rectified by the diode 350 and the capacitor Cl is applied to the sensor circuit 330. Accordingly, the noise component is not directly applied to the sensor circuit 330, thereby preventing deterioration of noise resistance.
In a case of focusing on a magnitude of noise applied to the sensor circuit 330, as shown in FIG. 7B, diodes are preferably connected to both of the plus-side path A and the minus-side path B.
However, if diodes are connected to both of the plus-side path A and the minus-side path B, forward drop voltages of the diodes are applied to both of the plus side and the minus side, thereby increasing the lowest operation voltage of the field device. Also, because rectifying is performed on both of the plus side and the minus side, a direct voltage is occurred between an nA point and an nB point in the figure, thereby causing a negative effect that it is necessary to increase a withstand voltage of a circuit inside of the field device.